This invention relates to an improved multi-level sealing method for effectively sealing electronic elements from moisture.
A variety of means and methods have been known in the art for preventing moisture from entering electrical elements. For example, preformed solid sealants have been designed, which melt, or are liquefiable upon the application of heat, and are hardenable again to a solid state. Such a sealant is disclosed in Siner, U.S. Pat. No. 3,077,639, which is shaped so as to allow air, caused to expand in the casing or electrical element being sealed upon the application of heat, to escape through the area being sealed prior to the sealing thereof. The air, therefore, escapes before the sealant covers the opening and formation of bubbles in the sealant is presumably prevented.
Other inventions have been patented which are designed to protect the electrical element from direct contact from a mold while sealant is being applied to the electrical element. Such patents are exemplified by Sternbeck, U.S. Pat. No. 3,165,568, which uses a "free form foil" and Schroeder, U.S. Pat. No. 4,374,080, which utilizes a "silicon mold", which, because of the design of the mold, allows the molds to be stacked. Because the mold is made of silicon rubber, no mold release agents are needed and, therefore, the metal electrical leads of the electrical device to be sealed are free to bond with the encapsulation or sealant material. Other inventions utilize a combination of pressure and vacuum to ensure a proper seal, free of bubbles. Such an invention is disclosed in Oldham, U.S. Pat. No. 4,681,718.
A drawback to the methods of sealing electrical elements, known in the art, is that water is drawn into the electrical element, through the seal, as the sealant cools. Additionally, with only one layer of sealant applied in a single application, water typically can seep through minute pathways in the sealant into the electrical element. Thus, there is a need in the art for providing a method of sealing electrical elements and the like, so that moisture is prevented from being drawn into the electrical element as the seal cools and that, once the seal is cooled, that moisture is prevented from seeping through the seal into the element. It, therefore, is an object of this invention to provide an improved multi-level sealing method for preventing moisture from being drawn into the element as the seal cools and for preventing seepage of moisture into the element after the seal has cooled.